The present invention relates generally to systems including filters and, more particularly, to an apparatus for indicating when to replace a filter in a system including an air filter.
Disposable air filters are commonly used in commercial and residential heating, ventilating and air conditioning (HVAC) systems, and in portable air purification units. Such filters typically include a frame, a fibrous non-woven filter material, and a reinforcing structure that supports the filter material. After a period of use, these filters become dirty or clogged and must be cleaned or replaced. Proper filter maintenance helps keep the HVAC equipment operating at maximum efficiency and reduces operating costs. Depending on the operating conditions, filter replacement may be required several times a year.
Filter replacement is typically done on a regular, fixed-interval basis. That is, the filter is replaced after a certain recommended fixed period of time, such as three months, has passed. This fixed period of time, however, may not be appropriate for a given situation. For example, the fixed time period may be too short, in which case the filter is discarded prematurely, or the fixed time period may be too long, in which case the filter is used beyond the time when it should have been changed. Individuals may also lose track of when the filter was last changed or may forget to change the filter at the appointed time. Changing the filter prematurely is wasteful and results in unnecessary expense, and waiting too long can result in system inefficiency or may negatively impact air quality.
Devices for indicating when to change or clean an air filter are known in the prior art. U.S. Pat. No. 4,321,070 (Bede), for example, discloses a whistle which, when installed in an air filter, emits a shrill tone to signal a predetermined clogged condition of the filter at which time the filter should be cleaned or replaced.
U.S. Pat. No. 3,071,914 (Gesmar) discloses a filter condition indicator for a filter fan using a differential pressure gauge. The differential pressure gauge includes a hollow indicator tube and a float that moves in response to the pressure differential. U.S. Pat. No. 5,668,535 (Hendrix et al) discloses a filter condition sensor and indicator in which a heated thermistor is positioned in a by-pass air flow path.
Residential HVAC systems and portable air purifiers often utilize high performance filters commonly referred to as panel filters or pleated filters. Such filters are available from 3M Company, St. Paul Minn. under the Filtrete™ brand. Such filters have a generally thin flat construction and include a somewhat rigid peripheral frame and an interior filtering region formed from a fibrous non-woven filter media. To improve the performance of such filters, the filter media may be electrostatically charged. Such filters come in a variety of sizes and may include a support structure formed of, for example, wire mesh, paperboard, chipboard or glue strips to provide additional support for the filter media. Even with such support structures, however, the interior filtering region of such filters tends to exhibit a certain degree of bending, flexing, bowing, or bulging when subjected to the load created by air passing through the filter.
Devices that detect a predetermined extent of filter budge to operate an end-of-life signal are also known in the patented prior art. U.S. Pat. No. 3,928,006 (Martineau) discloses a device for signaling dirty filters including a position sensitive electrical switch positioned to detect a predetermined extent of filter bulge, thereby to operate an end-of-filter-life signal. The operation of this device, however, is highly sensitive to the position of the device relative to the filter. To properly adjust the location of the device, a mounting structure is required.
There is still a need for a simple, durable, reliable, inexpensive system and/or device that can accurately indicate when to replace or clean a filter based on the degree of bowing or flexing of the filter. There is also a need for such a device that is less dependant on the precise positioning of the device relative to the filter. That is, there is a need for a filter change indicator that is relatively insensitive to where it is placed in relation to the filter and can, therefore, be positioned at different locations from the filter and still yield accurate output about when to clean or replace the filter.
It would be desirable to provide a small, simple, durable, inexpensive system and/or device that can accurately and reliably indicate to a user when to clean or replace a filter, such as an air filter. It would also be desirable to provide such a device that does not have to be positioned a precise distance from the filter to accurately indicate when to clean or replace the filter, and does not require calibration or repositioning depending on the type of filter used or the operating conditions.